In machine and system components that are subject to elevated temperatures and temperature variations during operation, temperature differences are encountered that vary with the operating conditions. In the steady-state condition these differences depend only on the operating conditions prevailing at the moment, while in the transient condition they also vary with the preceding operating conditions or the preceding component temperatures. These temperature differences cause thermal stresses that add to the stresses from external loads, such as individual forces, pressures, etc., and that often make up a significant part in the total stresses in the material. Additionally, elevated temperatures may severely reduce the strength of the material.
Both the instantaneous load on the material, once it exceeds certain values, and the cyclic load arising during the entire service period, i.e. Low Cycle Fatigue (LCF hereafter) will cause damage to the component. Both must therefore be monitored and if at all possible, considered in the open- or closed-loop control of the machine.
For this purpose, the material temperatures and the thermally induced stresses in the components must be determined as a function of the actual course of operation. Direct measurement of these parameters is often prevented, and especially with rotating components, temperatures and thermal stresses are very often impossible to measure. Yet it is difficult to determine them accurately from other, more readily available measureable parameters, especially if the components are complex in geometry and because temperature development within a component varies with time. Up to now there are no means known for determining the temperature distributions which are at the same time fast and precise enough for the said purpose. It is necessary to determine temperature and material stresses in real time as well for the open- and closed-loop control duty as for the monitoring purpose. For the latter, this is necessary because otherwise unacceptable data storage capacities must be provided. This is particularly the case for aircraft engines.
For stress analysis and in the determination of LCF effect in operation, centrifugal forces as well as the thermal stresses must be evaluated as a function of engine service. The only instantaneous operating condition data available are general flight and operating parameters, such as conditions at the engine inlet, rotor speeds and gas temperature in the primary flow downstream of the combustion chamber. Temperature measurements on the rotor are prevented. From the available parameters, the associated temperature distribution and thermal stress profiles must be determined.
U.S. Pat. No. 4,228,359 discloses means for controlling thermal turbomachines in which the rotor temperature is evaluated based on an assumption that the rotor is a one-dimensional heat conductor, namely, an infinitely long cylinder of locally constant heat transfer on the cylindrical surface (cf. column 16, lines 1 to 10). It has been found that this ignores the mutual dependence of the temperatures in the different regions of the rotor and does not permit an adequate and accurate determination of the actual temperature distribution in real-time operation. This is due to the fact that the rotors of turbines are components of essentially disk-shape basic configuration in which the heat transfer is considerable at the lateral faces. Since at any time, the temperatures of the working medium differ greatly along the lateral faces of the disk and additionally may vary from one lateral face to the other, and since the temperature distributions of the working medium along the lateral faces of the disk are additionally subject to considerable variation with time, the heat transfer at the lateral disk faces is a paramount factor for the temperature distribution in the rotor. Ignoring this temperature transfer, therefore, will produce considerable inaccuracies.